Frequency-responsive control apparatus for electric alternators

ABSTRACT

In automatic regulating apparatus for an electric alternator a main thyristor is connected in series with an excitation winding across the alternator terminals and is controlled by a sensing circuit to which a signal corresponding to alternator output voltage is passed through an inductor to give frequencyresponsive control.

United States Patent inventor Peter J. Wyles Stamford, England Appl. No. 746,140

Filed July 19, 1968 Patented Nov. 9, 1971 Assignee Newage Lyon Limited Stamford, England FREQUENCY-RESPONSIVE CONTROL APPARATUS FOR ELECTRIC ALTERNATORS 4 Claims, 1 Drawing Fig.

US. Cl 322/32,

322/24, 322/73 Int. Cl 1102p 9/30 Field 01 Search 322/28, 29,

[56] References Cited UNITED STATES PATENTS 3,238,439 3/1966 Hobbs et al 322/32 X 3,290,582 l2/l966 Roosma et al. 322/28 X 3,37l,266 2/1968 Davidson et al. 322/32 X 3,470,456 9/1969 Reinert 322/28 X Primary E.raminer-T. E. Lynch Assistant Examiner-H. Huberfeld Auorney Watson, Cole, Grindle & Watson ABSTRACT: In automatic regulating apparatus for an electric alternator a main thyristor is connected in series with an excitation winding across the alternator terminals and is controlled by a sensing circuit to which a signal corresponding to alternator output voltage is passed through an inductor to give frequency-responsive control.

PATENTFUNBV em V V 2,5,1

INVLNIOR TER Juom Hvuas RY MW 6%, j/wul AITORNEY FREQUENCY-RESPONSWE CONTROL APPARATUS FOR ELECTRIC ALTERNATORS This invention relates to control apparatus for electric alternators, of the type having a circuit including an excitation coil by which the excitation of the alternator is produced or con trolled, connected to the output of the alternator in series with a current control device, conveniently a thyristor, for controlling the current through the coil. The excitation coil may in some cases be the field winding of the alternator itself and in other cases it may be the field winding of an auxiliary exciter for the alternator.

The invention is particularly, though not exclusively, applicable to control apparatus of the type in which the terminal voltage of the alternator is compared with a constant datum or reference voltage to give an error signal. The error signal is applied to the input of a transistor powered by a clipped rectified voltage from the output of the alternator. The transistor controls the rate of charging of a capacitor during each cycle. The capacitor voltage controls the firing of a unijunction or double-base transistor, and the latter triggers the firing of a thyristor in series with the excitation coil, the latter being shunted by a freewheeling diode.

In order to derive the error signal from the sensed signal the sensed signal may be applied to a bridge network incorporating a zener diode.

According to the present invention the control device includes a sensing circuit to which a signal corresponding to alternator output voltage is passed, and an inductor connected in the input to the sensing circuit.

Thus the voltage delivered to the sensing circuit will be reduced by the drop due to the reactance of the inductor, thus providing a simple form of frequency-responsive control.

The invention may be put into practice in various ways but one specific embodiment will be described by way of example with reference to the accompanying drawing, in which the single F 1G. is a circuit diagram of an automatic thyristor control for an alternator.

US. Pat. No. 3,548,288, filed July 19, 1968, by the same inventor as this application, is directed to a bypass circuit for improving the excitation buildup of an alternator. Applicants US. Pat. No. 3,577,061, filed July 19, 1968, relates to another apparatus for providing excitation buildup of an alternator.

The control apparatus is of known type employing a thyristor in series with an excitation coil 11. In some cases this may be the actual field winding of the alternator but in the actual embodiment described it is the field winding of an auxiliary exciting generator which in turn supplies the field winding of the main alternator.

The excitation winding is connected in series with the thyristor across two phase terminals and 16, of the alternator 18, and is shunted by a diode l9, termed a freewheeling diode, which allows the circulating current to continue to flow through the excitation winding when the thyristor is switched off.

The thyristor 10 is provided with a firing circuit powered from a controlled supply represented by a zener diode connected in series with a resistor 21 across the same two phase terminals 15 and 16. The firing circuit comprises a unijunction transistor 22 having its bases connected across the zener diode in series with a pair of resistors 23 and 24, one base resistor 24 being connected between the cathode and trigger of the thyristor.

The same zener diode 20 is connected, through a diode 25, to a supply capacitor 26 providing a supply for a timing circuit. The timing circuit comprises a transistor 30 having its emitter connected through a variable resistor 31 to the positive terminal 32 of the supply capacitor and its collector connected to the unijunction transistor input, and also through a timing capacitor 33 to the negative terminal of the supply. The base is connected through a resistor 34 and capacitor 35 to the positive terminal 32 of the supply to give stabilization and in addition is connected through a resistor 36 to a sensing circuit.

The sensing circuit is supplied from an isolating voltage transformer 40 having a primary winding 41 connected across the same two phase terminals 15 and 16 and a secondary winding 42 connected to a bridge rectifier 43 the output of which is connected to the input of a bridge network 45 com prising three resistor arms 46, 47 and 48 and a zener diode 49. The bridge is further protected by a capacitor 44 connected across its input to clip any high transient voltages.

Connected in series with the secondary winding 42 and the input to the bridge rectifier 43 is an impedance unit 50 comprising an inductor 51 and variable resistors 52 and 53. Resistor 52 can be replaced, or short circuited, by a short-circuiting link if not required. The purpose of the inductor 51 is discussed later below.

The bridge network 45 is set by the variable resistors 52 and 53 to be balanced at a predetermined alternator output voltage, and any change of the alternator voltage from this value will produce an error signal in one direction or another, at the bridge output, which is connected between the base of the transistor 30 and the positive supply terminal 32.

This provides a very simple and convenient way of deriving an error signal, but it will be appreciated that other methods of doing this may be employed if preferred.

The inductor 51 provides a very simple form of frequencysensitive control. The inductor has a comparatively high reactance so as to drop a substantial portion of the voltage of the secondary winding 42, the amount depending on the frequency. Thus for example if the frequency falls the reactance of the inductors falls and the voltage applied to the network 45 is increased thereby causing the control circuit to reduce the excitation and lower the alternator output voltage. At this reduced level, however, the regulator still retains its close voltage regulating performance with variation of load. The ratio of the transformer 40 may be variable to enable an increased secondary voltage to be employed when the inductor 51 is included.

Withthe arrangement as so far described, difficulty may be experienced in obtaining satisfactory buildup of the alternator voltage. Thus when the alternator is started, the initial voltage is that due to residual magnetism and this may be insufficient to cause the thyristor 10 to conduct at all. To overcome this difficulty, the apparatus is provided with a bypass circuit to bypass the thyristor 10.

In its simplest form (not shown) this comprises simply a resistor which may be adjustableor variable in series with a diode connected in parallel with the thyristor 10. The value of the resistor is chosen to allow sufficient excitation current to flow, when the thyristor 10 is not conducting, to enable the voltage to build up to a value sufficient to operate the thyristor firing circuit. As soon as this occurs current begins to flow through the thyristor, and under normal operating conditions the amount of excitation current which continues to fiow through the shunt circuit is only a small proportion of the total and does not in any way upset the control exercised by the thyristor 10.

For some requirements the simple arrangement described may not give a sufficiently rapid buildup since if the value of the resistor is reduced too far it will render the control exercised by the thyristor 10 less effective.

In this case the thyristor 10 may be bypassed by a tapered resistor and diode 61, provided with a bypass thyristor 62 connected to bypass the diode 61 and excitation winding 11, and having its trigger and cathode connected respectively to one end and the tapping of the resistor 60.

The bypass resistor 60 may be of relatively small value so as to pass a useful current through the excitation coil in order to start the buildup of excitation, even at a very small value of alternator output voltage. As the latter builds up, and the current through the bypass resistor 60 and diode 61 also builds up, the bypass thyristor 62 begins to conduct so that the bypass current which bypasses the main current control thyristor 10 also bypasses the excitation coil. In other words practically the only current passing through the excitation coil is that passing through the control thyristor which therefore exercises effective control.

The bypass circuit may be connected directly in parallel with the thyristor 10, but preferably, as shown, the resistor 60 is connected to the neutral point 17 rather than to the phase terminal 15. This reduces the voltage applied to it and hence reduces the dissipation of power in the resistor 60.

A requirement may be that the machine should be capable of starting and building up its voltage with a permanently connected load, in which case the voltage produced by the residual magnetism is still further reduced, thus aggravating the difficulty of causing the firing circuit to operate.

To meet this difficult a readily saturable current transformer 70 is connected through a rectifier 71 to the excitation winding 11. The current transformer 70 is designed so that at a comparatively small current value it will produce sufficient excitation to provide a voltage which will start the firing circuit, but will then become saturated so that however much the load current increases there will be little further addition to the excitation. This not only has the advantage of providing the required initial excitation under the arduous conditions referred to, without interfering with the control by the thyristor under operating conditions, but it also means that the same current transformer can be employed as a standard item for alternators having a wide range of rated current outputs.

What we claim as our invention and desire to secure by Let ters Patent is:

1. Apparatus for controlling an electric alternator, comprisa circuit connected to the output of said alternator for ener gizing said alternator, current control means for cuit, means receiving an AC input signal corresponding to the alternator output voltage, said means is frequency sensitive progressively controlling said cirto provide an output signal inversely related to the speed of said alternator,

a sensing circuit responsive to said output signal for controlling said current control means whereby the alternator output voltage is reduced with increasing speed of said alternator and increased with decreasing speed of said alternator.

2. Apparatus for controlling an electric alternator as in claim 1 wherein said sensing circuit includes means for rectifying said output signal and means for providing a substantially constant reference voltage, and said output signal is an error signal generated by comparison of said substantially constant reference voltage with said rectified signal,

and said current control means includes current-generating means powered by a clipped rectified voltage from the output of said alternator and controlled by said error signal, means for storing a signal representative of said error signal and driven by said current-generating means, and means responsive to said stored signal to cyclically trigger said circuit.

3. Apparatus for controlling an electric alternator as in claim 2 wherein said circuit includes an energization coil, a thyristor having an anode and cathode serially connected to said excitation coil and a gate triggered by said trigger means, and said excitation coil is shunted by a diode.

4. Apparatus for controlling an electric alternator as in claim 3 wherein said current-generating means includes a transistor having emitter and collector electrodes powered by said clipped rectified voltage and a base electrode receiving said error signal, said means for storing is a capacitor connected to said collector electrode, and said trigger means is a unijunction transistor having a control electrode connected to said capacitor and a base electrode connected to said gate.

l l l i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,619,763 Dated November 9, 1971 Inventor(s) Peter J wyles It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1: Line 5 (Column 4) cancel reduced" and insert --increased--.

Line 6 (Column 4) cancel "increased" and insert --decreased--.

Signed and sealed this 17th day of April 1973.

(SEAL) Attest:

EDWARD M. PLETCHER,JR. ROBERT QOTTSCHALK Attesting Officer Commissioner of Patents QM 304050 USCOMM-DC scan-Pen a ".5. GOVERNMENT PRINTING OFFICE i II; 0-555-53. 

1. Apparatus for controlling an electric alternator, comprising; a circuit connected to the output of said alternator for energizing said alternator, current control means for progressively controlling said circuit, means receiving an AC input signal corresponding to the alternator output voltage, said means is frequency sensitive to provide an output signal inversely related to the speed of said alternator, a sensing circuit responsive to said output signal for controlling said current control means whereby the alternator output voltage is reduced with increasing speed of said alternator and increased with decreasing speed of said alternator.
 2. Apparatus for controlling an electric alternator as in claim 1 wherein said sensing circuit includes means for rectifying said output signal and means for providing a substantially constant reference voltage, and said output signal is an error signal generated by comparison of said substantially constant reference voltage with said rectified signal, and said current control means includes current-generating means powered by a clipped rectified voltage from the output of said alternator and controlled by said error signal, means for storIng a signal representative of said error signal and driven by said current-generating means, and means responsive to said stored signal to cyclically trigger said circuit.
 3. Apparatus for controlling an electric alternator as in claim 2 wherein said circuit includes an energization coil, a thyristor having an anode and cathode serially connected to said excitation coil and a gate triggered by said trigger means, and said excitation coil is shunted by a diode.
 4. Apparatus for controlling an electric alternator as in claim 3 wherein said current-generating means includes a transistor having emitter and collector electrodes powered by said clipped rectified voltage and a base electrode receiving said error signal, said means for storing is a capacitor connected to said collector electrode, and said trigger means is a unijunction transistor having a control electrode connected to said capacitor and a base electrode connected to said gate. 